This invention pertains to methods of making microstructured optical fibers.
Microstructured optical fibers are known. See, for instance, A. A., Abramov et al., Electronics Letters, Vol. 35(1), Jan. 7, 1999, pages 81-82; and R. P. Espindola et al., Electronics Letters, Vol. 35(4), Feb. 18, 1999, pages 327-328.
Briefly, microstructured fibers comprise features (exemplarily but not necessarily air filled holes) that are axially oriented and typically extend the length of the fiber. Such fibers can have unique properties for at least two reasons. First, a large refractive index difference (xcex94nxcx9c0.45) occurs at the internal air/silica interfaces. This can provide the fiber with optically inactive regions, and with large numerical aperture. Second, the holes can be filled with appropriate materials to actively control the optical properties of the fiber. See, for instance, A. A. Abramov et al., IEEE Photonics Technology Letters, Vol. 11(4), pages 445-447, April 1999. Microstructured fibers are advantageously used in optical devices and sensors, e.g., as optical bandgap material, dispersion compensating fiber, high intensity lasers and amplifiers, and continuum generation fibers.
Prior art microstructured fibers are typically made by a very labor-intensive method, comprising bundling together glass tubes and, typically, a glass core rod, to form the desired geometry. See, for instance, U.S. Pat. No. 5,907,652. The process frequently involves extensive handling of the assembly by the fabricator, frequently resulting in contamination of the assembly, and requiring several cleaning steps. Furthermore, prior art assemblies typically are relatively short (e.g., 0.3 m), compared to standard preform lengths (typically 1 m or more).
The prior art method of making microstructured fiber typically also comprises collapsing an overclad tube over the assembly, such that the rod and tubes are held together. The resulting preform is then drawn into fiber, typically under conditions such that the interstitial regions collapse, and the tubes remain open due to pressure that builds up in each separate tube.
It will be appreciated that the prior art method has shortcomings. For instance, there are only a few geometries (e.g., hexagonal) that are relatively easy to make with the prior art xe2x80x9cbundle and overcladxe2x80x9d technique. Furthermore, for microstructured fiber to be used in applications that require more than a few meters of fiber, it will be necessary to lower the content of impurities which affect the fiber""s background loss and strength. Still furthermore, it is difficult to make large preforms ( greater than 0.3 m, desirably xcx9c1 m) length by the prior art bundle and overclad method.
In view of the potential usefulness of microstructured fiber it would be desirable to have available a method of making such fiber that is not subject to, or at least less subject to, the shortcomings of the prior art method. For instance, it would be desirable to have a method that is less prone to contamination, and is less operator dependent. Furthermore, it would be desirable to have a method that is capable of making non-symmetrical microstructured fiber. This application discloses such a method.
All cited references are incorporated herein by reference.
The invention is embodied in an improved method of making microstructured optical fiber. The method comprises providing a vessel (exemplarily, but not necessarily, tube shaped) having a length and an inner diameter, and that furthermore comprises two or more elongate elements (exemplarily rods, tubes, wires or fibers) extending at least a portion of the length of the vessel and being maintained in a predetermined spatial arrangement with respect to the vessel. The method also comprises at least partially filling the vessel with the elongate elements therein with a silica-containing sol, and permitting or causing the sol to gel, such that a gel body results, with the elongate elements embedded in the gel. The method further comprises separating the gel body from the vessel and the elongate elements (exemplarily with the aid of a release agent), drying, purifying and sintering the gel body, and drawing the microstructured optical fiber from the sintered gel body.
In an exemplary embodiment of the inventive method the elongate elements are rods or rod-like objects including tubes, exemplarily glass rods or steel rods, that are maintained in the desired spatial arrangement by holding fixtures, exemplarily a bottom and a top end cap with appropriately located holes and recesses. The vessel typically is a tubular vessel, with the bottom opening of the vessel closed off by a removable cap or other appropriate closing means. The top holding fixture typically is axially movable to facilitate removal of the elongate elements from the gel body.
In a further exemplary embodiment of the inventive method the elongate elements are physically, chemically or thermally removable elongate elements, e.g., polymer rods or fibers, and the method comprises removing said elongate elements after gelation of the sol by, e.g., pyrolysis or chemical action.
It will be understood that the elongate elements need not to be of circular cross section, and need not all have the same sizes and/or shapes. Furthermore, it will be understood that the elongate elements need not be removed from the gel body all at the same time.
It is anticipated that the inventive method will be able to provide gel bodies of length and diameter similar to those of state of the art monolithic (i.e., without multiple through holes) gel bodies, and typically also longer and/or thicker than prior art microstructured fiber preforms.
It will be appreciated that the below described means for the practice of the inventive method are exemplary only, and that those skilled in the art will be readily able to design and build means that meet special requirements, e.g., particular arrangements of the elongate elements. Indeed, it will undoubtedly be appreciated that substantially any arrangement of the elongate elements can be produced with the same basic apparatus, for instance, by providing holding fixtures that reflect the desired arrangement of the elongate elements.
The method according to the invention frequently has further advantages over the prior art method of making microstructured optical fiber. For instance, the former lends itself to mass production of relatively large preforms. The former also uses relatively cheap raw materials that nevertheless have relatively high purity, approaching that of conventional optical fiber. The former also is substantially operator-independent.